The present invention relates to the method and apparatus for controlling defrost and refrigeration operations in a refrigeration system having a plurality of evaporators. More particularly, the invention relates to controls allowing the evaporators to be defrosted and refrigerated independently and without overloading the system. The invention has utility in refrigeration systems having hot gas or other defrosting equipment.
It is customary in large refrigeration systems such as found in food stores or supply houses to utilize an integrated refrigeration system for a plurality of refrigeration units. A common compressor and condenser supply all of the refrigerant needed by the evaporators in the various units. However a certain degree of independency of the units is required because of the different demands imposed on the units. For example, in a food store, a refrigeration cabinet containing ice cream or frozen foods is normally held at approximately -20.degree. F. and imposes far more stringent demands on the refrigeration system than a milk cabinet or vegetable tray that operates in the vicinity of +40.degree. F. Furthermore, temperature and humidity conditions and the frequency with which the cabinets are opened and closed at various times of the day also affect the refrigeration process. Because of the variations in demand, the accumulation of ice and frost on different evaporator coils in the system varies widely, and independent defrosting and refrigeration controls are needed to keep the evaporators and the system operating efficiently. In this respect, reference to an evaporator is intended to refer to one or more evaporator coils connected in parallel or series to be refrigerated and defrosted in common.
In large commercial refrigeration systems, defrosting is frequently accomplished by transmitting hot refrigerant in gaseous form directly through the evaporators to melt the accumulated ice and frost. Such hot gas defrosting systems may reduce the available refrigerant for cooling purposes to an inadequate level for other evaporators in the system unless control is exercised over the number of evaporators that can be placed in the defrosting mode at any one time. Also with other defrosting devices such as electrical heaters connected to the evaporator coils, the number of evaporators defrosted at any given time must be controlled to prevent power overloads and undue demands on the refrigeration equipment.
Accordingly, it is customary in refrigeration systems having multiple evaporators to limit the number of evaporators that are placed in the defrosting mode at any given time. Sucn limitations can be imposed by scheduling the defrost operations for particular times of day; however, timed defrosting may not be satisfactory where demands on the refrigeration system are irregular and incapable of prediction. In these cases, a demand defrost system is generally preferred. In a demand system, frost sensors associated with the evaporators provide signals when accumulated frost and ice has reached a given level. In the absence of further control, however, it is clear that a demand system could place more than one and possibly all of the evaporators in defrost at one time. To prevent this possibility, control systems have been devised such as disclosed in U.S. Pat. No. 3,894,404 and copending U.S. patent application Ser. No. 812,198 filed July 1, 1977 by Willitts and Zickwolf and entitled "Automatic Defrost Control for Refrigeration Systems". In the copending application, scanning means are provided to periodically interrogate or examine the frost sensors in a predefined sequence. When one of the sensors indicates that its corresponding evaporator is in need of defrost, scanning is halted until defrost of the evaporator is complete. A certain priority can be developed by increasing the frequency that one evaporator is scanned relative to the others. Thus, for example, a high priority evaporator can be scanned twice as often as the other evaporators to increase the probability of detecting the need for defrost at an earlier point in time.
It is an object of the present invention to provide a defrost and regrigeration system which allows a plurality of evaporators to be controlled independently of one another and which constitutes and improvement over the prior art systems.